High-voltage fuses are widely used by utility companies to protect branch circuits and various electrical equipment, including most frequently transformers. Usually, high-voltage fuses are elongated assemblies in which there is a fusible element surrounded by a material which will help cool and quench the arc resulting when the fusible element rapidly melts. Thus, such fuses often are filled with materials such as sand, boric acid, bone fiber or liquid solutions which will generate water vapor to help cool and quench the arc.
Fusing of the fusible element in a high-voltage fuse generates a considerable volume of high pressure gas and hot metals which are expelled violently from the fuse, which also cools and interrupts or breaks the arc between the terminals which are attached to the fuse. The elongated high-voltage fuse construction spaces the fuse terminals in order to make sustained arcing more difficult. Moreover, various mechanical schemes have been developed for separation of the fuse terminals upon melting of the fusible element. Thus, springs, gravity, explosive charges and the rapid and violent vaporization of arc quenching materials have all been used to separate fuse terminals on melting of the fusible element. Typical of such terminal separating schemes are the fuse cutout assemblies and current interrupters of U.S. Pat. Nos. 2,174,477, 2,315,320, 2,481,298, 2,516,026, 2,524,101, 2,989,608, 3,518,483, 3,644,791, 3,702,419, 3,889,222, 4,275,372, 4,318,150, 4,538,202, 4,626,955, 4,688,143 and 4,743,996.
In general, such fuse cutout assemblies and current interrupting devices pay little attention to the hazards caused by the rapid generation of hot gases and explosive ejection of hot metals and fuse components during generation of such gases. Hot gases, metals and components, however, pose a substantial safety problem in some installations and in the most typical application for fuse cutout assemblies, namely, pole-mounted protection of branch distribution systems and transformers, these hot materials pose a serious fire hazard. In California, for example, utilities are required to clear a space 10 feet in diameter around pole-mounted fuse cutout assemblies.
The problem of containing hot gases and metals on fusing of fusible links has been addressed in connection with isolators for lightening arresters. Lightening arresters are designed to discharge surge currents through the ground and interrupt the flow of dynamic or system currents along the path established by the surge current before the isolator assembly operates. If the lightening arrester does not function properly to cutoff the path to the ground after the surge current has been discharged, the system current follows the surge current to the ground. The isolator assembly, therefore, will heat up, fuse and blow open the circuit thereby clearing the system dynamic following current to the ground. As will be appreciated, however, the surge current which must be dissipated from a lightening strike is very substantial and rises very rapidly, making it extremely difficult for the fusible link to react fast enough and for the housing to contain fusible element components. Thus, a lightening arrester isolator must operate in a manner which is fundamentally different from a fuse cutout assembly. The isolator assembly fusible link must break the circuit almost instantaneously or else the high energy will essentially blow up the isolator. Fuse cutout assemblies, by contrast, are not exposed to such energy and can take longer to melt or fuse.
Bearing the distinction in mind between a fusible isolator assembly for a lightening arrester and a fuse cutout assembly, one isolator device which purportedly contains the resulting hot gases and metals generated when the fusible element melts is shown in U.S. Pat. No. 4,503,414. In this fuse-based isolator assembly, a shielding cup surrounds the fusible member, and upon melting of the fuse, the cup drops down to separate the fuse terminals. The cup also is positioned to catch and contain hot gases and metals produced upon melting of the fuse.
In practice, however, it has been found that the fusible element cannot react fast enough to open the circuit. The result is that the fuse explodes destroying both the spacer and cup, rather than containing the hot gases and metals. The rapid and high energy rise explosively blows the cup away from the remainder of the assembly so that any containment of the arc and hot gases is only temporary, and in every case, the hot shielding pieces are detached from the remainder of the assembly and present a fire hazard.
Accordingly, it is a primary object of the present invention to provide a fuse cutout assembly and method which will replace current liquid fuses, SMU fuses and open link fuses which are presently mounted on transmission line poles and used to protect transformers and branch distribution lines.
It is a further object of the present invention to provide a fuse cutout assembly and method which will be effective in quenching the arc and interrupting the electrical circuit, while at the same time containing substantially all of the hot gases, metals and debris generated during fusing of the fusible element.
Still another object of the present invention is to provide a fuse cutout assembly and method which is constructed in a manner capable of reliably absorbing substantial energy discharge as a result of rapid melting of the fusible element.
Still a further object of the present invention is to provide a fuse cutout assembly and method in which all components will remain attached together during fusing to reduce the fire hazard presented during rapid melting of the fuse.
Another object of the present invention is to provide a fuse cutout assembly and method which is suitable for pole-mounting and can be easily visually inspected from the ground to determine whether it is operable or inoperable.
Still another object of the present invention is to provide a fuse cutout assembly and method which is durable, economical to construct, can be easily retrofit to existing installations and has an improved reliability of operation.
The fuse cutout assembly and method of the present invention have other objects and features of advantage which will be set forth in more detail in the Best Mode of Carrying Out the Invention and will be apparent from the accompanying drawing.